Argonne Study Examines Impact of Real World Drive Cycles on Efficiency and Cost of Different PHEV Configurations

11 June 2009

Results from a study by Argonne National Laboratory on the impact of real world drive cycles on the fuel efficiency and cost of different plug-in hybrid electric vehicle (PHEV) configurations suggest that while different PHEV configurations all demonstrate great potential for displacing petroleum use (with fuel displacement increasing linearly with available electrical energy), the relative benefits of adding more battery capacity seem to decrease with increasing pack size.

Aymeric Rousseau, program manager at Argonne, presented a small slice of this wide study at the Advanced Automotive Battery Conference 2009 (AABC) this week in Long Beach.

In the segment of the study presented, the Argonne team modeled four PHEV configurations: an input power split with a fixed ratio between the electric machine and the transmission (e.g., Camry hybrid) PHEV with a 4 kWh and an 8 kWh pack; and a series hybrid (extended-range electric vehicle, e.g., Volt) with a 12 kWh and a 16 kWh pack. These were compared to a conventional HEV, using the same hybrid drive as the power split PHEV, and a conventional combustion engine vehicle.

For drive cycles, the Argonne team used the Kansas City data collected in 2005 by the US EPA, which instrumented more than 100 drivers and collected their driving statistics for one day.

The PHEVs used different control strategies:

EV/CS (Thermostat) strategy for the series configuration. Here the controller drives as long as possible using battery energy, depleting the state of charge (SOC) from 90% to 30%. The engine turns on only if the road load exceeds the power capability of either the battery or the motor. Once the battery reaches charge sustaining mode (regular hybrid mode), the engine is to regulate the SOC.

Load engine power strategy. An SOC-based power threshold is used to urn the engine on. As a result, the engine can be turned on during charge depleting mode. To maximize charge depletion, the engine only provides the requested wheel power without recharging the battery.

Optimum engine power strategy. Similar to the Load Engine strategy, the engine is turned on based on a threshold. Here, however, the controller attempts to restrict the engine operating region close to its peak efficiency. As a result, the engine might be used to recharge the battery during charge depletion.

Basic comparison of fuel consumption showed mean values of:

Basic conventional: 6.6 L/100km (35.6 mpg US)

Hybrid (HEV): 4.69 L/100km (50.2 mpg US)

Split 4 kWh: 3.27 L/100km (71.9 mpg US)

Split 8 kWh: 2.32 L/100km (101.4 mpg US)

Series 12 kWh: 1.50 L/100km (156.8 mpg US)

Series 16 kWh: 1.23 L/100km (191.2 mpg US)

The larger the battery, the more fuel saved. However, what we also noticed was that the delta for consumption is not linear. The fuel we save by going from 4 to 8 kWh is much greater than the fuel saved going from 12 to 16.

—Aymeric Rousseau

On top of the significant gain achieved by using a standard HEV compared to a conventional vehicle (27.6%), the 4 kWh configurations adds an additional 20%. The gains from adding further battery capacities decrease when going from 8 to 16 kWh, with only a 10% increase from 12 to 16 kWh. Looked at another way, 4 kWh of battery energy provides 50% of the fuel displacement gains achieved with a 16 kWh battery.

The Argonne team found similar electrical consumption across the PHEV options for short distances. The largest discrepancies are found with medium distances of 15-25 miles; these drive cycles are characterized by both low and large power demands. While the split 8 kWh option will have an engine on event, the series configurations will continue to operate in all electric mode.

Cost benefit analysis. Based on the results, Argonne also modeled out the cost-benefit of the different configurations. Some of those findings included:

Assuming an electrical cost of $0.09/kWh and a fuel cost of $4/gallon, the HEV breaks even at 7.5 years, while the PHEVs range from 8 to 12.5 years.

A longer daily drive distance can significantly reduce payback time. Based on the assumption considered, one should drive at least 30 miles per day to have an acceptable payback (4-6 yeas for small energy batteries and 6-8 years for larger battery energies).

HEVs are more cost-effective than the split 4 kWh for driving longer than 30 miles, but the order is reversed for shorter distances.

When driving long distances (> 40 miles), both series configurations achieve similar payback since the additional battery cost is offset by fuel efficiency benefits.

Compared to the HEV powertrain, payback is close to 8 years for low energy batteries and 11 years for larger batteries.

An increase in fuel price from $4 to $5/gallon decreases the payback time by one year on average.

The group is looking at additional drive cycles, Rousseau said.

Based on the assumptions considered, for the mid-term, the cost of PHEVs remains high, requiring further research and development for batteries and electric vehicles.

You get the payback for the country in less imported oil. I know the individual has a payback period, with car price and fuel price, but what is the payback for all of us with less imported oil? We do not have to go to war nor get jerked around by people that want to mess with us....priceless.

Agree with SJC on this. Payback or ROI for hybrids generally does not factor in the added cost of petroleum based economies. And the idea that we could be growing a new industry domestically (GM's Volt battery facility)adds to dollars remaining at home. These li-ion batteries are currently factored at the highest price per kWh they will ever be.

This Argonne Study confirms the Voltage design choices. The "sweet spot" for fuel and economic efficiency is 8 kwh USABLE, which is the choice taken for the the Volt EREV Series electric vehicle.

As prices for batteries drop, it still appears that the smart course is to make the EREV less expensive, rather than boosting the AEV range from 40 miles as the 80-20 rule seems to be prevalent in the actual driving patterns of the 100 drivers monitored.

Way to go Volt Engineers. Also Kudos to Dr. Frank at UC Davis for his pioneering work to determine that "sweet spot" for hybrid vehicles. It was he and his Graduate students who conducted the original studies to determine that 80% of USA drivers drove less than 40 miles per day, most of the time.

This study is third party validation of GM's product strategy and confirms their earlier publications on the topic. The GM SAE studies, published in 2008 (http://www.greencarcongress.com/2008/02/gm-study-shows.html) and earlier this year (http://www.greencarcongress.com/2009/05/gm-quantifies-20090516.html) show that basically, the E-REV (VOLT) will do much more than a blended PHEV converted from a regular full hybrid (plug in PRIUS). The Volt will use much less fuel, make less CO2, and will make less pollution (HC) than the PHEV Prius, especially when run in the real world, and especially as charging opportunities become more prevalent.

This study shows that Parallel Plug-in Hybrids are an evolutionary dead end. They achieve no economies of form, requiring both mechanical and electrical drive trains. A series hybrid can dispense with the mechanical drive train.

The Volt's failure lies in morphology. A commuter car does not need 5 seats and the shape does not have to slavishly follow the form of a standard sedan. A smaller vehicle with a smaller genset tucked away could achieve high fuel economy at lower cost.

The first 4kWh displaces the most fuel and offers the best bang for the buck. It makes more sense to sell FOUR Prius PHEV with 4kWh rather than ONE 16kWh Volt. This would displace 48 times more fuel.

This study confirms that the next step in PHEV is with the parallel-series PHEV hybrids.

Note that Series hybrid can not use a small battery pack because it won't have enough power. This study is a bit flaw because we do not have a 12kWh battery pack that can accelerate Volt size/weight car 0-60 in 9 second (premise of the comparison).

You are really stretching to try and make a point for a Plug in Prius, like somehow minimizing the use of batteries to displace fuel is a good thing.

In the process you are comparing one car against two car. Why would I drive two Prius when I need to go somewhere? And if I used two Prius' for my trip, I wouldn't displace any fuel. Logically, I would drive one Volt instead.

The object is electrification, and the Volt does more than a Plug In Prius.

One Volt displaces more fuel that one Prius.
Two Volts displaces twice as much as Two Prius.
You can fill in the rest, because that is the future.

Sadly, the end goal, to reduce oil imports and GHG emissions is not very relevant here.
Sales drivers are 1) affordability and 2) perceived utility/style (4 or 5 seats, range, convenience etc; i.e. a “normal” car).
Expecting the imported oil and GHG problems to incentivize the individual buyer when it will cost more than +20% is foolish.
The Prius succeeds only partly, at this time, because of cost; it is a desirable vehicle otherwise for way more people than it’s present 2% sales penetration.
The Volt will apparently have the same cost problem but may be a bit more desirable, at least for some, so it might add another 2% to the hybrid sales penetration.
The common, and I believe correct assumption is that vehicles that are much less desirable will not sell in quantities, regardless of price or fuel consumption.
So, while this site might have many readers that believe all (or many) people should be driving EV1s, Insight-1s and Aptera’s, that’s not likely to happen, whether such vehicles are available or not.

Li-Ion battery cost/performance is, apparently, one of the few (maybe only) technologies that can turn the tide.

Higher gas taxes can make that happen sooner – maybe with tolerable side effects, maybe not.

Cellulose biofuels and PHEVs can make a difference, 2% may go to 5% in the next few years. Cellulose E10 and E85 available everywhere with all new cars FFV could make quite a big difference. No silver bullet, but a lot of golden BBs.